I recently bought a Thunderbass amp threaded here Works if you shake it More general discussion on Thunderbass amps here: http://www.letstalkguild.com/phpBB2/viewtopic.php?f=7&t=4710
The amp showed up and, after a couple tubes changed, powered up nicely. In anticipation of rebuilding the power supply, I started to go over it and found that, although the amp arrived w/ 6L6s, it was wired for 8417s - like at least John K's and maybe others. Most particularly, the two large light blue cylinders - resistors - in the amp didn't match the 6L6 schematic.
The pinouts for the 8417 and the 6L6 are identical:
But, as shown in Guild schematics for the same amp, their electrical characteristics are different. Considering the prices of 8417s:
The Tube Store each 79.95
Toobey Doo each $54.95
Tubes and More each $54.55
Tubeworld pairs $110-$135, individuals $55-$60
The fact that JJ, ElectoHarmonix, Groove Tubes, and the others are not supporting the 8417, and the plentiful supply of 6L6s, I'm going to rewire it for 6L6s. The good news is that for the 6L6s to work correctly in an amp wired for 8417s, only a few changes are necessary.
The power supply and outputs from the 6L6 schematic are shown below. The red below indicates the primary high voltage DC which goes directly to the output transformer and then to the plates; where the amplified AC signal exits the tube. The green below is the screen voltage; the screens control the flow of electrons from the cathode to the plate and it is the flow of electrons that 'imprints' an identical copy of the AC waveform onto the plate but at a much higher amplitude; e.g., amplified. Finally the yellow indicates the bias wiring. The yellow should have been extended to indicate connection to the cathode - opposite the plate (too lazy to do the pic over )
The colored text boxes indicate the differences in electrical characteristics between 6L6- and 8417-wired amps with the 6L6s requiring higher plate and screen voltages but at only 40volts differential whereas the 8417s require less plate voltage but nearly 100volts differential between the plates and screens.
There are 4 blue circles around resistors with related text boxes. Three of the four resistors are direct swapouts; take out the resistor of 8417 value, replace with the 6L6 value. Simple enough; the remaining resistor to the far visual left will probably have to be adjusted to get 460 volts. Both schematics shown a 10 ohm/10watt resistor in that position. If the transformers are identical then slightly less resistance ought to generate higher B+. If not, it may not matter that much because what is more important is the voltage differential between the plates and screens; the absolute value of the B+ isn't that important; somewhat less voltage would only mean slightly less rated output.
So, other than possibly having to either calculate, or determine by trial and error, a value for the left-most resistor, changing 3 or 4 resistors will convert an 8417 amp to a 6L6 amp. The only other comment is that this power supply has gobs of capacitance; essentially reserve power to support higher volumes and lower frequencies; after all, it is a bass amp but with a guitar channel. No effort will be made to dirty up the guitar channel. The amp has .27uf coupling capacitors; large enough to knock down high frequency hash or grunge so that any effort to optimize the guitar channel will be mitigated by the coupling caps; if the caps are changed, the amp might be somewhat compromised as a bass amp.
Comments welcome; hold the jeering to a minimum.....
cj
The amp showed up and, after a couple tubes changed, powered up nicely. In anticipation of rebuilding the power supply, I started to go over it and found that, although the amp arrived w/ 6L6s, it was wired for 8417s - like at least John K's and maybe others. Most particularly, the two large light blue cylinders - resistors - in the amp didn't match the 6L6 schematic.
The pinouts for the 8417 and the 6L6 are identical:
But, as shown in Guild schematics for the same amp, their electrical characteristics are different. Considering the prices of 8417s:
The Tube Store each 79.95
Toobey Doo each $54.95
Tubes and More each $54.55
Tubeworld pairs $110-$135, individuals $55-$60
The fact that JJ, ElectoHarmonix, Groove Tubes, and the others are not supporting the 8417, and the plentiful supply of 6L6s, I'm going to rewire it for 6L6s. The good news is that for the 6L6s to work correctly in an amp wired for 8417s, only a few changes are necessary.
The power supply and outputs from the 6L6 schematic are shown below. The red below indicates the primary high voltage DC which goes directly to the output transformer and then to the plates; where the amplified AC signal exits the tube. The green below is the screen voltage; the screens control the flow of electrons from the cathode to the plate and it is the flow of electrons that 'imprints' an identical copy of the AC waveform onto the plate but at a much higher amplitude; e.g., amplified. Finally the yellow indicates the bias wiring. The yellow should have been extended to indicate connection to the cathode - opposite the plate (too lazy to do the pic over )
The colored text boxes indicate the differences in electrical characteristics between 6L6- and 8417-wired amps with the 6L6s requiring higher plate and screen voltages but at only 40volts differential whereas the 8417s require less plate voltage but nearly 100volts differential between the plates and screens.
There are 4 blue circles around resistors with related text boxes. Three of the four resistors are direct swapouts; take out the resistor of 8417 value, replace with the 6L6 value. Simple enough; the remaining resistor to the far visual left will probably have to be adjusted to get 460 volts. Both schematics shown a 10 ohm/10watt resistor in that position. If the transformers are identical then slightly less resistance ought to generate higher B+. If not, it may not matter that much because what is more important is the voltage differential between the plates and screens; the absolute value of the B+ isn't that important; somewhat less voltage would only mean slightly less rated output.
So, other than possibly having to either calculate, or determine by trial and error, a value for the left-most resistor, changing 3 or 4 resistors will convert an 8417 amp to a 6L6 amp. The only other comment is that this power supply has gobs of capacitance; essentially reserve power to support higher volumes and lower frequencies; after all, it is a bass amp but with a guitar channel. No effort will be made to dirty up the guitar channel. The amp has .27uf coupling capacitors; large enough to knock down high frequency hash or grunge so that any effort to optimize the guitar channel will be mitigated by the coupling caps; if the caps are changed, the amp might be somewhat compromised as a bass amp.
Comments welcome; hold the jeering to a minimum.....
cj